In the two-stroke engines known at present, the piston opens and shuts the aspiration inlet; therefore, the control of the mixture flow is symmetric relative to the two return positions.
The angle between the beginning of the opening of the aspiration inlet and the return position of the piston is equal to the angle between the return position and the end of the closure of the aspiration inlet.
In typical two-stroke engines the full angle of the opening of the aspiration inlet is contained within 100.degree. and 110.degree. of the cam shaft rotation; the half of the angle corresponds to the rotation of the piston in the external direction and the other half corresponds to the return of the piston.
It has been observed that the symmetrical control of the flow of fuel to the crankcase is not a good solution as the aspiration inlet should open much earlier and close later and this is not possible with the traditional piston position.
To avoid this inconvenience a solution has been adopted, according to which circulation of gas is made independent from the piston movements with a rotary throttling valve mounted on the cam shaft.
This valve allows a symmetrical inflow of fuel to the crankcase while it is opened and shut by a rotary plate.
This plate is cut accordingly to the angle corresponding to the angle of the opening of the aspiration inlet and the inflow of fuel to the crankcase is dependent on the size of the aspiration inlet.
Other known solutions allow automatic control of the inflow of fuel by the alteration of pressure in the crankcase.
For this purpose plate valves are applied that consist of springing plates interconnected in parallel.
Due to the pressure within the crankcase, these plates move away proportionally to the depression value which allows the fuel inflow.
When the balance of pressures is achieved the plates shut the aspiration inlet without allowing the fuel to return.
A system of this kind is based on a mode of mechanical control of the inflow of fuel to the crankcase and, in the case where rotary plates are used, the construction of the engine is complicated, which increases costs of production.
Another disadvantage of these systems arises from the fact that their accurate functioning takes place only if the r.p.m. value of the cam shaft rotation is strictly specified, which renders them virtually inapplicable; therefore, such systems practically are not employed in circuit racing engines and rally engines.
In the case where the plate valves are used, proper functioning and capacity of those depend on the materials from which they are manufactured.
Nevertheless, considering the fact that the plates are mounted rigidly, they do not sustain prolonged strain and are deformed and worn out, which causes breakdown that, in turn requires frequent replacements.
Additionally, operation of the plates prohibits, due to internal reactions, the slow inflow of fuel to the degree in which the increasing resistance of the plates does not allow the fuel to inflow when it is most demanded.
The above solutions have other disadvantages, such as unsolved problem of uneconomical fuel consumption and high degree of environmental pollution which are caused by the two-stroke engines.
However, the pollution produced by two-stroke engines is generally caused by faulty fuel supply during the work cycle.
Actually, the combustion chamber is flooded with fuel that cannot be fully consumed, i.e. burnt up, during the only two strokes of the cycle.
Due to the above, the application of two-stroke engines has been significantly reduced lately in spite of their cost-effective construction and economical operation.
U.S. Pat. No. 4,356,798 discloses an aspirating thermodynamic valve capable of controlling the flow of fuel mixture and air from the carburettor to the crankcase of a two-stroke engine. Said valve comprises a hollow body suitable for being placed between the joint of the crankcase and the joint of the carburettor, along with suitably interpositioned seals, a bush, the extremity of which is placed inside of said body and cut out according to a predetermined inclination, and comprising a wall which is hinged by one end and is capable of closing said extremity.
U.S. Pat. No. 5,390,633 discloses the arrangement of an aspirating thermodynamic valve between the crankcase of an engine and the carburettor, and exhibiting a bush, the extremity of which is placed inside of the crankschaft intake body and is cut out according to a predetermined inclination, and comprising a wall which is hinged by one end and is capable of closing said extremity.
DE-A-3,741,880 suggests a valve in which a wall is made of ferromagnetic material and an electromagnet is mounted on the valve body. In this case the attraction force is designed for opening the valve and not for returning the wall in the position of closure as described in the present invention.